**Aviation, Space and Defence Glossary**

**Above sea level (ASL)**

Distance of the aircraft above mean sea level

**Absolute.**

A standard, fixed reference, as opposed to moving reference

**Acceleration.**

Rate of change of velocity, either scalar or vector, often with subscripts such as ENU or XYZ to denote the coordinate frame; time derivative of velocity; time integral of jerk

**Acceleration east.**

Aircraft acceleration in true east direction

**Acceleration north.**

Aircraft acceleration in true north direction

**Accelerometer.**

An inertial device for measuring acceleration, usually in three orthogonal axes (lateral X, longitudinal Y, and vertical Z)

**Accuracy.**

Measure of exactness, possibly expressed in percent

**Acquire.**

To begin reception of useful data

**Activate.**

To begin performing a mission objective, such as flying along a radial of a radio station; usually refers to a mode of radio navigation, such flying along that radial after capturing that radial

**ADC.**

Air data computer

**Advisory.**

A signal to indicate safe or normal configuration, condition of performance, operation of essential equipment, or to attract attention and impart information for routine action purposes

**AFCS.**

Automatic flight control system

**AGR slant range.**

Straight-line distance from the aircraft to a point on the ground

**Aiding.**

A process by which one or more sensors provide data to another sensor to produce results better than any single sensor; aiding occurs at the data source level or at the physical device level, depending upon specific implementation of the device and the data source

**Air Data Computer (ADC).**

A primary navigation data source. A navigation sensor based on atmospheric data sensors; usually measures static pressure, dynamic pressure, and outside air temperature; sometimes computes other atmospheric data, such as indicated airspeed, Mach number, calibrated airspeed As a guidance mode, ADC is least accurate of the listed modes and is used only as a last resort

**Air-ground ranging (AGR).**

Straight-line distance from the aircraft to a point on the ground

**Air-mass flight path angle.**

Angle in vertical plane of earthspeed vector and groundspeed vector; occasional definition for flight path angle

**Aircraft (ac).**

A craft that flies in the air; either fixed or rotory wing.

**Airy.**

A standard model for computing earth data

**Alignment.**

A basic control to a data source from controls and displays to align a device. Also a procedure to align physical devices, usually navigation sensors, so that they provide the most accurate results possible; commonly required by INS, AHRS, barometric altimeter

**Altimeter.**

A device to measure altitude, either barometric altitude or radar altitude

**Altitude.**

Height, usually with respect to the terrain below (radar altitude, feet above closest dirt) or fixed earth reference (barometric altitude, feet above mean sea level)

**Altitude error.**

A basic output from guidance to flight director, indicating the difference between actual altitude and desired altitude

**Altitude integral gain (KALTINT).**

A guidance control law parameter, generated by the vertical guidance modes; varies control authority of the altitude integral in vertical guidance, to reduce steady-state errors in altitude error

**Altitude integral input (ALTINTVAL).**

A guidance control law parameter, generated by the vertical guidance modes; a reference altitude for reducing steady-state errors in altitude error

*Typical Units:*ft;*Dimensions:*Length**Altitude integral limit (INTMAGLIM).**

A guidance control law parameter, generated by the vertical guidance modes; limits the magnitude of altitude integral value

*Typical Units:*ft;*Dimensions:*Length;**Angle of attack.**

The difference between pitch and the air-referenced flight path angle; the angle between the aircraft center line and the airspeed vector in the vertical plane, positive when the nose is up;

*Symbols:*alpha;*Typical Units:*rad, deg;**Angular acceleration.**

Rate of change of angular velocity, either scalar or vector, often with subscripts such as XYZ to denote the coordinate frame; time derivative of angular position; time integral of angular acceleration;

*Symbols:*alpha;*Typical Units:*rad/s-squared;*Dimensions:*1/Time-squared;**Angular position.**

Amount of rotation about an axis, either scalar or vector, often with subscripts such as XYZ to denote the coordinate frame; time integral of angular velocity;

*Synonyms:*angle;*Symbols:*theta;*Typical Units:*rad, deg;**Angular velocity.**

Rate of change of rotation about an axis, either scalar or vector, often with subscripts such as XYZ to denote the coordinate frame; time derivative of angular position; time integral of angular acceleration;

*See Also:*tachometer;*Symbols:*omega;*Typical Units:*rad/s, rpm;*Dimensions:*1/Time;**Annunciator.**

Any one of warning, caution, or advisory;

*Synonyms:*alert;**Aperiodic.**

A process that executes based on events rather than a fixed rate, it is not synchronized to other processes of interest;

*Compare:*periodic;**APPR.**

Approach

**Approach (APPR).**

To fly towards a point; a basic guidance mode, providing lateral guidance, longitudinal guidance, and vertical guidance to a point at an operator selected groundspeed and radar altitude;

*See Also:*waypoint approach, ILS approach, rendezvous approach;**Approach to hover (AHOV).**

Hover approach of a rotary wing aircraft.

**Arm.**

To strive for a mission objective, such as flying toward a radial of a radio station; usually refers to a mode of radio navigation, such as striving to reach a specific radial of a radio station prior to flying along that radial;

*See Also:*capture criterion;*Compare:*activate, capture;**ASL.**

Above sea level

**Astronomical latitude.**

Latitude measured with respect to vector of apparent gravity;

*Compare:*geocentric latitude, geodetic latitude;*Symbols:*Phi sub A;*Typical Units:*rad, deg,DMS;**ATC.**

Air Traffic Control

**Atmospheric data.**

Environmental data related to the atmosphere at some point of interest

**Attitude.**

The primary aircraft angles in the state vector; pitch, roll, and yaw;

**Attitude Heading Reference System (AHRS).**

Combines information from a Magnetic Heading Sensor with self-contained aircraft acceleration data to provide attitude, heading, position, body inertial velocity, and body inertial acceleration. Typically a low-accuracy, self-contained navigation source using strapdwon accelerometers;

**Australian National.**

A standard model for computing earth data

**Automatic Direction Finding (ADF).**

A basic guidance mode, providing lateral guidance to a radio station. Equipment that determines bearing to a radio station;

**Automatic flight control system (AFCS).**

An automated system for controlling the primary flight controls, often with built-in functions for guidance and flight director, and sometimes radio navigation; many flight control systems include basic instruments similar to a AHRS; many flight control systems accept flight director inputs so that its radio navigation, guidance, and flight director can be bypassed

**Autopilot.**

A mode of an automatic flight control system which controls primary flight controls to meet specific mission objectives, such as maintain a heading or altitude

**Averaging filter.**

A filter for combining multiple data sources, usually of the same type, by adding with weighted averages; a simple average of the data sources

**Axis.**

One direction in an orthogonal reference frame;

**Azimuth.**

An angle in the horizontal plane, usually measured with respect to body coordinates

**Band-pass filter (BPF).**

A filter that allows frequencies between two cutoff frequencies to pass while attenuating frequencies outside the cutoff frequencies; a band-pass filter can be constructed as the composition of a low-pass filter and a high-pass filter;

**Bank angle.**

The angle between the horizontal plnase and the right wing in the lateral plane, positive when the right wing is down

**Barometric altitude.**

Height with respect to fixed earth reference (above mean sea level)

**Barometric altitude select (BALT SEL).**

A basic guidance mode, providing vertical guidance to an operator selected barometric altitude;

**Barometric pressure.**

Height with respect to fixed earth reference (barometric altitude, feet above mean sea level)

**Beacon.**

A device, usually based on the ground that aids in determining position or direction;

**Bessel 1841.**

A standard model for computing earth data

**Bias.**

An offset applied to a measurement for error correction

**Body.**

The aircraft, usually referring to a coordinate system;

**Body coordinates.**

Coordinates referenced to the body of the aircraft

**Boresight angle.**

The angle between the center line of a sensor and aircraft center line, either by design or by misalignment;

**Boresighting.**

A basic control to a data source from controls and displays to boresight a device; Also, a procedure to align the center line of physical devices, usually update sensors, so that they provide the most accurate results possible; a basic control to a data source from controls and displays

**Built-in simulation (BIS).**

Function in avionics software that simulates sensors, aircraft, and pilot, to exercise avionics software (including navigation, radio navigation, guidance and flight director); BIS is often used by a development team to check basic operation following installation of new software or patches; BIS is seldom used by aircraft flight crews or maintenance crews;

*Compare:*real-time engineering simulation;**Calibrated airspeed (CAS).**

Indicated airspeed corrected for instrumentation errors, but not for air density

**Calibration.**

A basic control to a data source from controls and displays for calibrating a device

**Caution.**

A signal which alerts the operator to an impending dangerous condition requiring attention, but not necessarily immediate action (from MIL-STD-1472D); an annunciator that is more critical than an advisory but less critical than a warning;

**Circular error probability (CEP).**

A probability that a percentage of two-dimension measurements will lie within a circle of given radius, with the circle centered at truth or mean of the measurements;

*Compare:*radial error probability, spherical error probability; CEP specifies test cases for measurement errors of sensors of two dimensions, such as velocity east and north.**Collective.**

A flight control operated by moving up or down with hand in rotary-wing aircraft, primarily to control lift (altitude); controls collective (total) pitch of the rotors on a rotary-wing aircraft;

**Communications.**

How well equipment is communicating;

*Values:*operational, degraded, failed**Complementary filter.**

A filter in which the complement of the filter is desired, giving the effect of a high-pass filter by implementing a low-pass filter

**Computer cycle.**

In a periodic, cyclical computer system, the most basic, fastest timing loop

**Continuous time.**

Time which can have any point expressed as a real quantity, without regard for any specific interval or processing rate

**Continuous-time equation.**

A mathematical relationship to describe a function of time, expressed in terms of continuous time

**Control law.**

The mathematical definition of a system used to control or to change the dynamic response of a system;

**Control surface.**

An airfoil attached to an aircraft that is moved to control the attitude of the aircraft; an surface to control flight of an aircraft indirectly, such as a swashplate to control pitch of rotor blades

**Controlled altitude (CTALT).**

A guidance control law parameter, generated by the vertical guidance modes; altitude that is being controlled

**Controlled speed (CTS).**

A guidance control law parameter, generated by the longitudinal guidance modes speed that is being controlled. Describes operation of flight director in which automatic flight control system causes flight controls to follow commands from flight director or errors from guidance

**Course cut limit (CCLIM).**

A guidance control law parameter, generated by the lateral guidance modes; limits the intercept angle of the flight path with a desired course, typically 45deg

**Cutoff frequency.**

The frequency at which the gain of a filter is at an edge of a band, usually taken to be when gain is 0.5, or -3.01dB; the frequency at which the output of a filter is half the power of the input

**Damped frequency.**

The frequency of oscillation of an underdamped second-order filter;

**Damping ratio.**

Control parameter for a second order filter.

*Symbols:*zeta;**Data source object (DSO).**

Software that receives data from a physical device, translates the data into standard units, maintains equipment status, and provides a common interface for each variation of a particular device;

**Data Transfer System (DTS).**

A device for transferring data with avionics, similar to a diskette drive;

**Dead reckoning (DR).**

A method of navigation based on basic information (barometric altitude, magnetic heading, airspeed, wind conditions) from best available source; sometimes short for air data dead reckoning;

**Depart from hover (dhov).**

A guidance mode providing lateral guidance, longitudinal guidance and vertical guidance for a set heading or bank angle, a set speed, and a set climb rate, altitude or pitch;

**Desired altitude (DESALT).**

A guidance control law parameter, generated by the vertical guidance modes; altitude which controlled altitude is attempting to

**Desired path.**

A trajectory in space determined by guidance to meet the current mission objectives;

**Desired speed (DESS).**

A guidance control law parameter, generated by the longitudinal guidance modes; speed which controlled speed is attempting to achieve

**Difference equation.**

A mathematical relationship to model a discrete function, expressed in terms of other values in the sequence; Difference equations are usually derived from differential equations.

**Differential equation.**

A mathematical relationship to model a continuous function, expressed in terms of derivatives; Initial conditions are usually given or implied. In avionics, differential equations are commonly used by systems engineers to model avionics systems. The systems engineer usually converts differential equations to difference equations for specification and implementation in software. In avionics, a differential equation usually models continuous-time phenomenon in terms of time derivatives

**Digital Map Generator (DMG).**

Digitial equipment that produces map video, and sometimes contains TRN; uses Digital Terrain Elevation Data and Digitized Feature Analysis Data; Displays reconstructed digital map data, aeronautical charts or photographs. The digital map data can be annotated with natural and man-made (point and linear) features as well as threats. A Terrain Referenced Navigation algorithm correlates altitude readings with digital map data to provide position data.

**Digital Terrain Elevation Data (DTED).**

On a digitial map, data for elevation of terrain; DTED is usually provided by Defense Mapping Agency

**Discrete time.**

Time divided into quantized intervals; in avionics, time is usually divived into equal intervals to create a periodic process

**Discrete-time equation.**

A mathematical relationship to describe a function of time, expressed in terms of discrete time

**Distance Measuring Equipment (DME).**

Equipment for measuring distance, usually from an aircraft to a ground station; usually part of a Tactical Air Navigation system

**Doppler.**

A technique for measuring velocity by radiating and determining frequency shift

**Doppler Navigation System (DNS, DPLR).**

A navigation for measuring velocity by radiating and determining frequency shift;

**DPLR.**

Doppler navigation system

**Drag.**

Force of air against aircraft acting in opposite direction of the airspeed vector projected into horizontal plane

**Drift.**

Slow, monotonic change in measured data

**Earth coordinates.**

Coordinates referenced to the earth

**Earth data.**

Environmental data related to the earth at some point of interest; usually a function of latitude and longitude

**Earth eccentricity.**

A measure of the degree to which the earth is oblate;

*Symbols:*epsilon;**Earth model.**

The earth model computes data related to the earth. Most data is a function of position.

**Earth radius.**

Radius of the earth, function of position, separate radii for longitudinal radius and for lateral radius

**Earth-referenced flight path angle.**

Angle in vertical plane of airspeed vector and groundspeed vector; usual definition for flight path angle

**Earthspeed.**

Total velocity measured with respect to a plane tangent to the earth's surface at the current position; a vector composed of velocity north, velocity east, and vertical velocity

**Elevation.**

An angle in the vertical plane through a longitudinal axis; height above mean sea level, usually of terrain;

**Elevator.**

A control surface on fixed-wing aircraft, usually mounted on the aft edge of stabilizers, that controls pitch, and is controlled by the yoke

**Equipment status.**

Operational status of a piece of equipment consisting of a status indicator and status words;

**Expanding square search.**

A pattern of progressively larger squares (a ``square spiral'') followed for searching the ground from an aircraft

**Extrapolate.**

Function to determine values from two or values in a table, when the given value lies outside of the range of the table; usually linear but can be higher order;

**Filter.**

A device to alter a signal; software to alter a data steam

**First-order filter.**

A filter in which the output follows the input, only more slowly; It is usually implemented in software as a difference equation of period T. The first-order filter is commonly used in avionics to smooth data, and to wash out transients at mode change. It is also used as a low-pass filter. When implementing a second-order filter on normalized variables, such as angles, the discontinuities require special treatment.

**Fix.**

A determination of one's position based on external data, such as a known terrain point

**Flight control system (FCS).**

A primary flight control system or an automatic flight control system;

**Flight controls.**

Controls in a cockpit for flying an aircraft; primary flight controls are wheel, yoke, cyclic, pedals, throttle, and collective; secondary flight controls are flight controls other than primary, such as flaps, slats, stabilizer, and landing gear;

**Flight path angle (FPA).**

Angle in vertical plane of earthspeed vector and groundspeed vector (usual definition), earth-referenced flight path angle; angle in vertical plane of airspeed vector and groundspeed vector (occasional definition), air-mass flight path angle

**Forward-Looking Infrared (FLIR).**

Sensor equipment used to supplement AGR, extend the aircraft visual search capability and provide position information for guidance and navigation update capability. Imagery derived from the FLIR sensor is displayed in the cockpit. FLIR pointing can be controlled manually using a tracking handle or automatically by the Mission Computer. FLIR converts a heat image into a video image and determines azimuth, elevation, and sometimes range of a point

**Gaussian radius of curvature.**

Radius of the earth for the best fitting sphere at a given position

**Geocentric latitude.**

Latitude measured with respect to horizontal through mass center of the earth